Determination of the $ \eta$ mass with the Crystal Ball at MAMI-B

Abstract.A new precise determination of the $ \eta$ meson mass is presented. It is based on a measurement of the threshold for the $ \gamma p\rightarrow p\eta$ reaction using the tagger focal-plane microscope detector at the MAMI-B facility in Mainz. The tagger microscope has a higher energy resolution than the standard tagging spectrometer and, hence, allowed an improvement in the accuracy compared to the previous $ \eta$ mass measurement at MAMI-B. Special emphasis was put on a very careful energy calibration of the electron beam and the tagging device, which reduced considerably the systematic uncertainty compared to the previous MAMI experiment. The result $ m_{\eta}=(547.865 \pm 0.031_{stat.}\pm 0.062_{syst.})$ MeV agrees very well with the precise values of the NA48, KLOE, CLEO and COSY-ANKE Collaborations and deviates by $ 6\sigma$ from the smaller value obtained by the GEM Collaboration at COSY.